Regioselective Polymerizations of Α-Olefins with an Α-Diamine Nickel Catalyst Cite This Article As: Liao Heng, Gao Jie, Zhong

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Regioselective Polymerizations of α-Olefins with an α-Diamine Nickel Catalyst Liao Heng, Gao Jie, Zhong Liu, Gao Hai-Yang, Wu Qing

Cite this article as: Liao Heng, Gao Jie, Zhong Liu, Gao Hai-Yang, Wu Qing. Regioselective Polymerizations of -Olefins with an -Diamine Nickel Catalyst[J]. Chinese J. Polym. Sci, 2019, 37(10): 959-965. doi: 10.1007/s10118-019-2227-y

View online: https://doi.org/10.1007/s10118-019-2227-y

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https://doi.org/10.1007/s10118-019-2227-y Chinese J. Polym. Sci. 2019, 37, 959–965

Regioselective Polymerizations of α-Olefins with an α-Diamine Nickel Catalyst

Heng Liao, Jie Gao, Liu Zhong, Hai-Yang Gao*, and Qing Wu School of Materials Science and Engineering, PCFM Lab, GD HPPC Lab, Sun Yat-sen University, Guangzhou 510275, China

Electronic Supplementary Information

Abstract Polymerizations of linear α-olefins (CnH2n, CH2＝CH―R, R = Cn−2) catalyzed by early transition metals typically afford amorphous polymers with alkyl chains (Cn−2), while chain-straightening polymerizations of α-olefins with nickel-based catalysts produce semicrystalline polyolefins. Polymerizations of various α-olefins were carried out using an α-diamine nickel catalyst with a significantly distorted chelating ring. The influences of temperature, monomer concentration, and chain length of α-olefins on polyolefin microstructure were examined in detail. The α-diamine nickel catalyst realized highly regioselective 2,1-insertion of α-olefins regardless of reaction temperature and monomer concentration. Increased chain length of α-olefins led to the formation of more linear polyolefin. Semicrystalline polyolefins with high melting temperatures (Tm) were made from α-olefins through highly regioselective 2,1-insertion and precise chain-straightening.

Keywords Nickel catalyst; α-Olefin; Polymerization; Regioselectivity; Chain walking

Citation: Liao, H.; Gao, J.; Zhong, L.; Gao, H. Y.; Wu, Q. Regioselective polymerizations of α-olefins with an α-diamine nickel catalyst. Chinese J. Polym. Sci. 2019, 37, 959–965.

INTRODUCTION mer branching structures including branching distribution and density are closely related to the regioselectivity that in- Precise control of polyolefin microstructure by olefin volves insertion fashion of α-olefin (1,2- or 2,1-insertion) coordination polymerization, especially branching structure, and chain walking process.[33−38] Coates’ groups have deeply is challenging and increasingly attractive because polyolefin studied a mechanistic model for polymerizations of α-ol- branching architecture is closely related to its properties and efins using α-diimine nickel catalysts and quantified eight [1−10] applications. Polymerization of α-olefins (CnH2n, different insertion pathways.[39] Like early transition metals CH2＝CH―R, R = Cn−2) catalyzed by early transition metals (Ti and Zr), direct insertion of α-olefin using nickel and pal- (Ti and Zr) usually affords polymers with alkyl chains (Cn−2) ladium catalysts in 1,2-mode without chain walking affords [11−13] regardless of 1,2- or 2,1-insertion of α-olefins. For polymers with alkyl chains (Cn−2). 1,2-Insertion of α-olefin example, 1-hexene and 1-octene are used as comonomers for and subsequent β-H elimination followed by metal migra- the synthesis of linear low density polyethylene (LLDPE) tion up to the primary carbon atom lead to 2,ω-enchainment [14−16] with butyl (C4) and hexyl (C6) branching, respectively. to give methyl branch (C1) in the polymer chain, while 2,1- Oligomerization of 1-decene with metallocene catalysts insertion of α-olefins and chain straightening result in produces poly(α-olefin) (PAO) as high-quality synthetic 1,ω-enchainment to give linear polymer chain without lubricants.[17,18] branches.[40−47] Besides, insertion of α-olefins into the sec- However, late transition metal catalysts, especially α- ondary carbon (the penultimate chain end position and sec- diimine nickel and palladium catalysts, possess distinctive ondary positions on the polymer chain) introduces unique al- chain walking characteristic.[19−32] The metal centers mi- kyl branches. The numerous combinations of insertion and grate along the polymer chain in the chain growing process chain walking pathways give rise to complex branching mi- through β-H eliminations, alkene rotations, and then reinser- crostructures of polyolefin. Therefore, precise control of tions into the metal-H bond. In terms of α-olefins polymeriz- polyolefin microstructure by nickel and palladium catalyzed ation with α-diimine nickel and palladium catalysts, poly- α-olefins polymerization remains a great challenge. In addition to the influence of polymerization conditions

* Corresponding author: E-mail [email protected] such as temperature and monomer concentration on the in- Invited article for special issue of "The 100th Anniversary of the Birth of sertion mode and chain walking pathways, the employed Prof. Shi-Lin Yang" catalyst structure plays a more crucial role. Currently, few Received December 6, 2018; Accepted January 21, 2019; Published online nickel catalysts have been reported to show high selectivity February 25, 2019 for α-olefin polymerization.[33,43,44,47−50] The first regiose-

© Chinese Chemical Society Institute of Chemistry, Chinese Academy of Sciences www.cjps.org Springer-Verlag GmbH Germany, part of Springer Nature 2019 link.springer.com 960 Liao, H. et al. / Chinese J. Polym. Sci. 2019, 37, 959–965 lective catalyst is the aminobis(imino)phosphorane nickel compounds were carried out under an atmosphere of dried system, which can produce poly(α-olefin)s with methyl and purified nitrogen with standard vacuum-line, Schlenk, or branches at well-defined intervals through 2,ω-enchainment glovebox techniques. [33] involving 1,2-insertion of monomer and chain walking. Materials Classic α-diimine nickel catalysts usually show poor se- The α-diamine nickel complex used in this study was pre- lectivity for α-olefin polymerization, but the catalyst modi- pared according to literature procedures.[58] Dichloromethane fications on backbone and N-aryl substituents can improve was distilled from P O under nitrogen, and toluene from the selectivity. Coates’ groups have reported that “sandwich” 2 5 Na/K alloy. Modified methylaluminoxane (MMAO, 7 wt% arylnaphthyl-α-diimine nickel catalysts produce chain Al in heptane) was purchased from Akzo-Nobel and used as straightened poly(α-olefins) with high melting temperatures received. Polymerization grade propylene was used directly of 113 °C via a combination of regioselective 2,1-insertion without further purification. 1-Hexene, 1-octene, and 1- and precision chain walking.[48] C -symmetric α-diimine 2 dodecene were purchased from Alfa Aesar Chemical, dried nickel catalysts are capable of catalyzing the regio- and iso- over CaH , and distilled under nitrogen before use. selective polymerization of 1-butene to produce isotactic 4,2- 2 poly(1-butene) with precision methyl branching.[49] Our Propylene Polymerization group has reported that a bulky camphyl α-diimine nickel A round-bottom Schlenk flask with stirring bar was heated catalyst could produce more linear polyolefins with repeated to 150 °C for 3 h under vacuum, cooled to room temperature, methylene units at high temperatures.[51,52] and then set to the desired temperature. Toluene of calculated Recently, our group has focused on the development of volume was transferred into the flask, and MMAO was [N,N] bidentate nickel and palladium catalysts for olefin added by syringe. For polymerization at −60 and −40 °C, polymerizations.[24−26,50−61] In addition to the steric and elec- propylene (10 g) was condensed into the flask by liquid tronic modifications of ligand frameworks, we have deve- nitrogen firstly, and polymerized as liquid monomer. For loped three kinds of nickel and palladium catalysts bearing polymerizations above −20 °C, the system was set at 0.05 α-diimine, amine-imine, and α-diamine ligands based on the MPa of propylene pressure. After 30 min, a nickel complex effect of combination fashions between metal and nitrogen solution in CH2Cl2 (2 mL, 20 μmol) was added to initiate the donor atom (Scheme 1). The [N,N] bidentate ligands with polymerization. After a certain polymerization period, the different hybrids can potentially impose distinct influence on polymerization was terminated by adding acidic methanol. the metal in regard of both polymer structure and reactivity The resulting precipitated polymers were collected, treated control for ethylene and α-olefin polymerizations. Unlike α- by filtration, washed with ethanol several times, and dried in diimine nickel catalyst, amine-imine nickel catalyst bearing vacuum at 60 °C. 2 two different coordinating functionalities (imine (sp N) and Polymerization of Liquid α-Olefins 3 amine (sp N)) exhibits a distinctive influence on regiose- A round-bottom Schlenk flask with stirring bar was heated lectivity involving insertion fashion in the polymerization of to 150 °C for 3 h under vacuum and then cooled to room [50] α-olefin. A bulky α-diamine nickel catalyst recently de- temperature. A desired amount of freshly distilled α-olefin veloped by our groups showed strong chain walking ability was charged into the flask, which contained the required in ethylene polymerization to afford highly branched PEs amount of toluene (if needed) and activator. After 30 min of [58] even at −60 °C and high ethylene pressure. Inspired by equilibrating period at the desired temperature, polymeri- the remarkable chain walking ability and the distorted co- zation was started by injecting a nickel complex solution of ordination geometry of α-diamine nickel catalyst, α-olefin CH2Cl2 (2 mL, 20 μmol) into the reactor. After a certain polymerizations catalyzed by the α-diamine nickel catalyst polymerization period, the reactions were terminated by under various conditions were conducted and reported here. adding acidic methanol. The resulting precipitated polymers Semicrystalline polyolefins were made using the α-diamine were collected and treated by filtration, washed with ethanol nickel catalyst by regioselective polymerization of α-olefins. several times, and dried in vacuum at 60 °C. Characterization EXPERIMENTAL The 13C-NMR data of polyolefin samples were obtained on a General Considerations Varian Mercury-Plus 500 MHz spectrometer at 110 °C, in o- All manipulations involving air- and moisture-sensitive C6D4Cl2 solution, and using 30 ppm for main chain of PE as

N N N HN NHHN Ni Ni Ni Br2 Br2 Br2 a-Diimine Ni Amine-imine Ni a-Diamine Ni 1 N: sp2, sp2 N: sp2, sp3 N: sp3, sp3 This work

Scheme 1 [N,N] bidentate nickel complexes with different combination fashions

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Liao, H. et al. / Chinese J. Polym. Sci. 2019, 37, 959–965 961 a reference. Branching content and distribution, as well as diamine nickel-catalyzed ethylene polymerization, which the fraction of 2,1-insertion were calculated according to the was attributed to the significantly distorted chelating ring literature.[37,51] Molecular weight and molecular weight structure of α-diamine nickel catalyst.[58] distribution of the polyolefins were determined on a PL-220 instrument at 150 °C, and 1,2,4-trichlorobenzene was employed as the eluent at a flow rate of 1.0 mL/min; data were 20 °C processed using narrow polystyrene standards. Differential scanning calorimetry (DSC) analysis was conducted with 0 °C a PerkinElmer DSC-7 system at a heating or cooling rate of 10 °C/min and DSC curves were recorded at the second −20 °C heating scan from −100 °C to 140 °C.

RESULTS AND DISCUSSION −40 °C α-Diamine nickel complex 1 was prepared according to pre- −60 °C viously reported method.[58] After activation with MMAO, α- diamine nickel 1 was firstly used to catalyze propylene poly- −50 0 50 100 150 merization. Table 1 shows the propylene polymerization Temperature (°C) results by α-diamine nickel catalyst 1 in a temperature range Fig. 1 DSC curves of PPs obtained by α-diamine nickel catalyst 1 from −60 °C to 20 °C. Generally, α-diamine nickel shows low activity towards propylene polymerization and the inc- Besides, the branching degree increased slightly and the reasing temperature led to an increase in catalytic activity. melting pointing of the polypropylenes decreased with in- Besides, α-diamine nickel 1 is less active than α-diimine creasing temperature. The percentage of 2,1-insertion calcu- nickel analogue (Scheme 1) for propylene polymerization, lated by the branching density clearly showed a downtrend at and this trend is consistent with ethylene polymerization.[58] elevated temperature, which strongly indicated that 2,1-in- Herein, we focused on the effect of temperature on micro- sertion of propylene is thermodynamically favorable for α- structure of the obtained polymer. diamine nickel catalyst. 13C-NMR analysis showed that 1H-NMR spectroscopy analyses of polymeric products methyl fraction and long branches fraction decreased with showed that the obtained polypropylenes (PPs) had very low the reducing temperature, further supporting this claim (Fig. branching degree (51−72/1000C) relative to the theoretical S1, in electronic supplementary information, ESI). value, which was a result of 1,3-enchainment arising from In addition to the effect of temperature, monomer concen- 2,1-insertion of propylene monomer and chain straightening. tration has an important influence on α-olefin polymeriza- DSC curves of the obtained polypropylenes also show the tions.[48,65] Therefore, polymerizations of 1-hexene under wide melting endotherms (Fig. 1). The melting temperature various conditions (temperature and concentration) were con- (Tm) was attributed to the methylene sequences in the po- ducted because monomer concentration of liquid 1-hexene lyolefin. Usually, lowering the temperature leads to a de- could be easily controlled. crease in chain walking in α-diimine nickel-catalyzed pro- As shown in Table 2, α-diamine nickel catalyst 1 shows pylene polymerization. For example, Brookhart type α- comparable activity to amine-imine nickel catalyst diimine nickel catalyst produced PPs with syndiotactic struc- (Scheme 1),[50] but lower activity than α-diimine nickel ana- [62−64] [35,40] ture at −78 °C. A C2-symmetry α-diimine nickel cata- logue for 1-hexene polymerization. Reaction tempera- lyst reported by Coates afforded isotactic PPs by 1,2-inser- ture in the 1-hexene polymerization presented the same ef- tion of monomer at −60 °C.[43,65] Differently, α-diamine fect on polymer microstructure as observed in propylene- nickel 1 still exhibited good regio-control for PP polymeriza- polymerizations. At the specified monomer concentration of tion by 2,1-insertion of monomers and chain straightening. 1.06 mol/L, the branching degree of poly(1-hexene) in- Even at very low temperature of −60 °C, α-diamine nickel creased with the increasing polymerization temperature, but catalyst exhibited strong chain walking ability, and 2,1-inser- 2,1-insertion percentage of monomer and melting tempera- tion of propylene led to 1,3-enchainment to form long ture of polymer decreased. These results further confirmed methylene sequences (Scheme 2). This temperature effect on that the temperature effect on α-diamine catalyzed α-olefin chain walking was similar to the previous observation in α- polymerizations was attributed to the catalyst nature.

Table 1 Propylene polymerization results by α-diamine nickel catalyst 1 a a b c d Entry T (°C) t (h) Yield (mg) Mn (kg/mol) PDI Br (/1000C) 2,1-Ins. (mol%) Tm (°C) 1 −60 20 52.1 10.8 1.73 51.3 84.7 92.5 2 −40 20 95.5 13.8 1.86 53.1 84.1 91.7 3 −20 12 168.3 19.7 2.06 61.2 81.7 88.2 4 0 4 73.0 11.6 1.52 69.5 79.3 83.0 5 20 4 118.5 23.0 1.36 72.1 78.4 71.5 Polymerization conditions: 20 μmol catalyst (in 2 mL CH2Cl2), [Al(MMAO)]/[Ni] = 200, toluene, total volume: 30 mL, 10 g of liquid propylene for entries 1 and 2, 0.05 MPa propylene pressure for entries 3−5. a Determined by high temperature GPC against polystyrene standard; b Determined by 1H-NMR spectroscopy; c Calculated by equation: 2,1-ins. (mol%) = 100(333-Br)/333; d Determined by DSC, broad endotherms

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3 2 Chain walking Ni 3 1 2,1-Insertion P 2 Ni (> 78%) 1 1,3-Enchainment 2 P 3 1 ΔT m n HN NH 1 Ni 1,2-Insertion P Ni 2 (< 22%) 3 Scheme 2 Different pathways in propylene polymerization catalyzed by α-diamine nickel catalyst 1

Table 2 1-Hexene polymerization results by α-diamine nickel catalyst 1 a b b c d e Entry T (°C) cH (mol/L) Yield (mg) Mn (kg/mol) PDI Br (/1000C) 2,1-Ins. (mol %) Tm (°C) 1 20 1.06 187.3 30.7 1.70 32.6 80.5 96.6 2 35 1.06 222.1 32.3 1.38 36.5 78.1 89.7 3 50 1.06 157.3 32.3 1.39 39.7 76.2 86.0 4 65 1.06 104.5 36.9 1.52 43.8 73.8 84.4 5 20 0.53 104.3 14.4 1.74 32.7 80.4 96.4 6 20 2.12 307.4 51.0 1.66 33.1 80.1 96.2 7 f 20 5.33 414.2 66.2 1.81 33.8 79.7 95.5 a Polymerization conditions: 20 μmol catalyst in 2 mL CH2Cl2, [Al]/[Ni(MMAO)] = 200, toluene, total volume: 30 mL, 8 h. cH: Concentration of 1-hexene; b Determined by high temperature GPC against polystyrene standard; c Determined by 1H-NMR spectrum; d Calculated by equation: 2,1-ins. (mol%) = 100(167 − Br)/167; e Determined by DSC; f 8 mL of 1-hexene monomer, no toluene solvent

The effect of monomer concentration was further studied catalyst, but they suffer from catalyst decomposition at high at 20 °C (Table 2). Increasing monomer concentration from temperature and low activity at low monomer concentration. 0.53 mol/L to 5.33 mol/L led to the enhanced activity and In comparison with previous reports on the synthesis of polymer molecular weight because of the increased insertion semicrystalline poly(α-olefins) with nickel catalysts,[48,66,67] rate of monomer. Surprisingly, the microstructures of poly(1- α-diamine nickel catalyst can yield semicrystalline polyol- hexene)s in terms of the branching degree and melting tem- efins with nearly the same Tm at high monomer concentra- perature remained nearly unchanged in a range of monomer tion even without solvent. Therefore, α-diamine nickel cata- concentration from 0.53 mol/L to 5.33 mol/L. The obtained lyst is undoubtedly an attractive candidate for the synthesis poly(1-hexene)s showed low branching degree of ~33/1000C of semicrystalline polyolefins from α-olefins because of the and high melting temperature of ~96 °C (Fig. 2). This obser- significantly enhanced activity and productivity. vation was obviously different from the previous report on α- diimine nickel-catalyzed α-olefin polymerizations.[40,48,66,67] Merna and Shiono reported that lowering the concentrations of α-olefin led to the decreased branching degree but increased Tm of polyolefins for α-olefin polymerizations with 5.33 mol/L α-diimine catalysts.[66,67] Coates reported that sandwich-type α-diimine nickel catalyst showed improved regioselectivity 2.12 mol/L [48] and precise chain walking at a low monomer concentration. 1.06 mol/L Generally, decreasing the monomer concentration would slow down the insertion rate of monomer. Therefore, chain 0.53 mol/L walking rate increased relative to the insertion rate of monomer, which caused more monomer enchainment and re- −100 −50 0 50 100 150 duced branching degree. However, our α-diamine nickel Temperature (°C) catalyst system showed a unique effect of concentration, Fig. 2 DSC curves of poly(1-hexene)s catalyzed by α-diamine which resulted from the nearly full monomer enchainment. nickel catalyst 1 at different monomer concentrations Because the 2,1-insertion selectivity (~80%) was independent of monomer concentration, the 2,1-insertion of 1-hexene Polymerizations of α-olefins with different chain lengths into α-diamine nickel catalyst 1 would fully lead to 1,6-en- were carried out to evaluate the effect of chain length on mi- chainment prior to the insertion of next monomer.[58] There- crostructure of polyolefins (Table 3). With the increased fore, the branching degree of 33/1000C and melting tempe- chain length of α-olefins, the branching degree decreased rature were independent of the monomer concentration of 1- and Tm value increased consistently. Since 2,1-insertion of α- hexene. To the best of our knowledge, the α-diamine nickel olefin would lead to 1,ω-enchainment by chain-straighten- catalyst is the first example that makes monomer enchain- ing process, increased chain length of α-olefin would pro- ment insensitive to monomer concentration. Increasing tem- duce longer successive methylene (―(CH2)n―) sequences in perature and reducing monomer concentration can improve the polymer chain. As shown in 13C-NMR spectra of poly(α- the monomer enchainment of α-olefins for α-diimine nickel olefin)s (Fig. 3), long methylene sequences along with a

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Table 3 Polymerization results of α-olefins catalyzed by α-diamine nickel catalyst 1 a a b c Entry Monomer Concentration (mol/L) Yield (mg) Mn (kg/mol) PDI Br (/1000C) Tm (°C) 1 d Propylene (P) − 118.5 23.0 1.36 72.1 71.5 2 1-Hexene (H) 5.33 414.2 66.2 1.81 33.8 95.5 3 1-Octene (O) 4.25 338.2 58.1 1.66 31.5 101.1 4 1-Dodecene (D) 3.01 330.5 50.3 1.65 24.4 111.3 5 e H-O-D 4.21 296.4 60.8 1.54 34.8 101.1 a Polymerization conditions: 20 μmmoL catalyst (in 2 mL CH2Cl2), [Al(MMAO)]/[Ni] = 200, 8 mL monomer, no solvent, 20 °C, 8 h. Determined by high temperature GPC against polystyrene standard; b Determined by 1H-NMR spectrum; c Determined by DSC; d 0.05 MPa propylene pressure, toluene used, total volume 30 mL, 4 h; e Equal volume (2.67 mL) for each monomer, total monomer volume 8 mL small amount of methyl and long chain branching are clearly straightened poly(α-olefin)s reported up to date, and α-diam- observed at 30.0, 19.99, and 14.09 ppm, respectively. DSC ine nickel 1 enabled highly regioselective 2,1-insertion and curves also prove the formation of long methylene se- precise chain walking. Importantly, α-diamine nickel 1 could quences on the basis of an obvious melting peak (Fig. 4). achieve high regioselectivity at high monomer concentration That is to say, polymerizations of longer chain α-olefins with with high activity and high productivity in comparison with those of the reported α-diimine nickel catalysts. In industry, α-diamine nickel 1 afforded more linear PEs with higher α-olefins were prepared as a mixture of different carbon melting temperature (T ). Polymerization of 1-dodecene pro- m numbers. To assess the practicality of the process, a mixture of duced the polyolefin with Tm of 111.3 °C, which was re- 1-hexene, 1-octene, and 1-dodecene was polymerized by garded as linear low-density polyethylene (LLDPE). This 1/MMAO (entry 5 in Table 3). The polymerization afforded Tm of 111.3 °C reached the maximum value of chain semicrystalline polyolefin with a melting point of 101.1 °C (Fig. 4). This result suggested that semicrystalline polyol-

(CH2)n efins could be synthesized from mixed α-olefin feeds, redu- ≈ αB1 cing the need for tedious industrial separation processes. βB1 brB1 brBn 1B1 n > 4 2B 3Bn n CONCLUSIONS 1Bn αB1 βB1 1B PH brB1 1 1Bn In conclusion, we report that α-diamine nickel catalyst with a distorted chelating ring converts α-olefins into semicrystalline polymers with Tm > 100 °C. The α-diamine nickel PO brBn 3B 2B n n catalyst is highly regioselective for 2,1-insertion of α-olefins followed by precise chain straightening. Increased polymeri- PD zation temperature led to more branched polyolefins with reducing Tm because 2,1-insertion of α-olefins was therm- P(H-O-D) odynamically favorable for α-diamine nickel catalyst. Monomer concentration had no influence on polyolefin 50 45 40 35 30 25 20 15 10 microstructure, and the branching degree decreased and Tm δ (ppm) increased consistently with the increased chain length of α- Fig. 3 13C-NMR spectra of poly(α-olefin)s obtained by α-diamine olefins. A mixture of α-olefins was used to generate nickel catalyst 1 substantially linear polyolefins with α-diamine nickel catalyst at high monomer concentration. α-Diamine nickel catalyst is an attractive candidate for the synthesis of semicrystalline polyolefins from α-olefins in industry. PH

Electronic Supplementary Information PO Electronic supplementary information (ESI) is available free of charge in the online version of this article at http://dx.doi.org/10.1007/s10118-019-2227-y. PD ACKNOWLEDGMENTS P (H-O-D) This work was financially supported by the National Natural −100 −50 0 50 100 150 Science Foundation of China (Nos. 21674130, 51873234), Natural Temperature (°C) Science Foundation of Guangdong Province (No. 2017A030310 Fig. 4 DSC curves of poly(α-olefin)s obtained by α-diamine 349), Fundamental Research Funds for the Central Universities (No. nickel catalyst 1 17lgjc02), and PetroChina Innovation Foundation (No. 2017D-

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